A vacuum-assisted, countergravity casting process of the mold immersion type is described in such prior patents as the Chandley et al U.S. Pat. Nos. 4,340,108 issued July 20, 1982, and 4,606,396 issued Aug. 19, 1986. That countergravity casting process involves providing a mold having an expendable porous, gas permeable upper mold member (cope) and an expendable lower mold member (drag) engaged together, sealing the bottom lip of a vacuum chamber to the mold such that the vacuum chamber confronts the gas permeable upper mold member, submerging the bottom side of the lower mold member in an underlying molten metal pool and evacuating the vacuum chamber to urge the molten metal upwardly through one or more ingate passages in the lower mold member and into one or more mold cavities formed between the upper and lower mold members. Initial solidification of the molten metal is typically effected in the mold ingate passages, which are sized to this end, to permit withdrawal of the mold from the molten metal pool even through the metal filling the mold cavities may still be molten and unsolidified. In this way, the overall casting cycle time is significantly reduced. The above-described vacuum-assisted, countergravity casting process has been characterized by high production rates and high yields, especially in the casting of thin-walled parts (e.g., cast parts having a wall thickness of 0.5 inch or less), and has provided designers of cast parts with the opportunity and incentive to become more aggressive in calling for thin-walled cast parts as a means of reducing part weight.
Such expendable countergravity casting molds are complex in that in addition to the mold cavities themselves, they also require sealing surfaces thereon as well as means for securing the molds to the vacuum chamber. They are typically relatively expensive (e.g., compared to green sand molds) in that they require more expensive materials (e.g., resin-sand mixtures and curing agents therefor).
Moreover, the inventors have discovered that as the wall thickness of some parts cast in such molds is reduced to lower part weight, the presence of minute, non-metallic inclusions (e.g., non-metallic inclusions less than 0.010 inch diameter) in the microstructure become more significant and are deleterious to the mechanical properties (e.g., strength) and performance of the cast part.
This capability of such minute inclusions to adversely affect the mechanical properties and performance of thin-walled cast parts made by the above-described casting process has not only inhibited to some extent use of this casting process but also has placed an additional burden on the casting inspection equipment and procedures of the foundry. In particular, such minute inclusions typically cannot be detected in thin-walled cast parts using conventional x-ray equipment heretofore used. As a result, more sophisticated and costly defect analysis equipment, such as tomography or real time, high sensitivity x-ray analysers, must be used in an attempt to detect the presence of such harmful inclusions in the cast part. The cost of producing acceptable, thin-walled cast parts is thus increased.
In order to take full advantage of the vacuum-assisted, countergravity casting process described hereinabove in the casting of thin-walled parts, there is a need to reduce the costs of the process and to minimize the presence of objectionable inclusions in the resulting cast parts without adversely affecting the high production rates and yields achievable with this casting process.
It is an object of the present invention to provide an improved vacuum-assisted, countergravity casting apparatus and process that satisfies these needs.
It is another object of the invention to provide an improved vacuum-assisted, countergravity casting apparatus and process that enable the casting of a plurality of thin-walled parts without compromising the mechanical properties and performance of the cast parts in service.
It is still another object of the invention to provide a vacuum-assisted, countergravity casting apparatus and process for simultaneously casting a plurality of thin-walled metal parts in expendable casting molds carried on a common drag slab that is immersible in an underlying molten metal pool during casting and that includes an individual slab ingate passage cooperatively registering with one or more mold ingate passages in each casting mold for supplying molten metal to each casting mold in controlled manner from the underlying molten metal pool.
It is still another object of the invention to provide a vacuum-assisted, countergravity casting apparatus and process of the preceding paragraph wherein each slab ingate passage preferably includes molten metal filtering means therein for removing inclusion-forming impurities from the molten metal as it is drawn upwardly into each casting mold to minimize the presence of harmful inclusions in the cast part that can adversely affect part performance.